Recent research has revealed that genetic recombination plays a role in the regulation of gene expression and cellular differentiation as well as its long known role in the inheritance of all individuals. The precise relationships of homologous and site-specific recombination, and the degree of their respective roles in gene expression and inheritance are not known, but rapid progress is being made in the study of both types of recombiation in vitro. Studies of purified E. coli recA protein have shown that it is an ATPase that catalyzes homologous pairing in two experimentally distinguishable steps: synapsis which puts molecules in homologous register by a side-by-side interaction of a single-strand with duplex DNA, and strand transfer, which requires a free end and produces heteroduplex joints by a concerted and polar mechanism. Since the recombination proteins of phage Lambda, the exonuclease and Beta protein, can replace the recA function in E. coli, we supposed that the phage Lambda proteins, Beta protein in particular, might promote homologous pairing. Preliminary experiments suggest that Beta protein, which lacks detectable ATPase activity, can promote the homologous pairing of duplex molecules with each other, as well as the pairing of complementary single strands and the pairing of a single strand with duplex DNA. Using a specific antiserum of Beta protein, and mutant forms of the protein, we will try to prove that Beta protein is responsible for the observed effects. We will study the effect of Beta protein on the interaction of single-stranded DNA with duplex DNA with the aim of looking for a combined action of Beta protein and Lambda exonuclease in th production of heteroduplex joints. With regard to the possible pairing of duplex molecules, we will look for other factors, endonucleases or topoisomerases, that may initiate actual strand exchanges. We will also try to isolate recombination intermediates from cells in which the Lambda recombination proteins, exonuclease and Beta protein, are promoting the recombination of duplex molecules of phage PhiX174. The in vivo and in vitro studies may provide complementary information on the mechanism of recombination promoted by the Lambda system. As we gain insight into the functional interactions of Beta protein and Lambda exonuclease, we will focus more on their physical interactions.